If the population of the world continues to increase at the present rate of 76,000,000, persons a year as U.N. statistics show, it is quite probable that the mankind will confront a serious food crisis even within this century as the prospects for food supply in the future are very bleak.
Efforts for increasing the sources of protein in the fields of agriculture and fisheries are subject to certain restrictions as it is hardly possible to drastically expand the acreage of agricultural cultivation or the fishing grounds and also because the yields of agricultural and marine products are greatly affected by weather and, further, a drop yield is unavoidable in future cultivation as the area for such cultivation is being cornered into an unfavorable environment due to steady expansion of population and industrial areas. Under such circumstances, high hopes are placed on utilization of single cell protein which can be produced in commercial quantity and with high efficiency in all seasons of the year without being affected by weather and also without causing natural environmental disruptions.
Heretofore, saccharides, sulfite pulp waste liquor and such have been used as base materials for producing microbial cells such as yeast cells, and interest has been aroused recently in normal paraffin which can be obtained from petroleum fractions at relatively low cost, and efforts are being made in many countries for producing the microbial cells using such normal paraffin as a carbon source to produce so-called petroleum protein. However, use of such normal paraffin as a base material for production of microbial cells also involves many difficult problems such as listed below.
(1) Since normal paraffin is almost insoluble in the culture medium, means for facilitating its culture, such as high speed agitation, addition of emulsifying or dispersing agents, large-scale aeration, etc., are required, resulting in increased consumption of energy.
(2) As much heat energy for fermentation is generated, high cooling cost is encountered.
(3) High refining cost is required for removing harmful substances such as 3,4-benzpyrene.
(4) It is not easy to remove normal paraffin from the produced microbial cells.
In an attempt to overcome such problems, use of acetic acid, ethanol, waste materials from processing of agricultural and marine products and the like as base material (carbon source) for production of microbial cells has been proposed. However, these materials still have many difficultires with regard to cost and stable supply. Methane and carbon dioxide are also noticed with interest for use as the base material because they can be supplied in abundance at low cost, but use of these materials is attended by the problems that growth rate of the microorganisms is unsatisfactory and that cost for production equipment is high because these materials are gaseous form.
In view of the foregoing, we have studied carbon sources for production of microbial cells and reached the conclusion that methanol is free of the difficulties such as mentioned above and therefore best suited for use as the carbon source. Methanol can be produced in commercial quantity from petroleum, coal and natural gas, so that its supply is stable and it is also obtainable at relatively low cost. Further, as methanol is soluble in water, it is suited for use as carbon source in the culture medium of microorganisms.
We then pursued a search for microorganisms which are effectively capable of assimilating methanol as a carbon source.
Mold, yeast and bacteria are known as typical examples of microorganisms which are capable of assimilating methanol as a sole carbon source. However, mold and yeast are lower in growth rate than bacteria, and also yeast requires expensive growth factors, so that these substances are disadvantageous for producing the preferred microbial cells economically. It is to be also noted that both mold and yeast are lower in crude protein content, which is an important factor in determining the nutritive qualities of the microbial cells, and also lower in sulfur-containing amino acid content in cell protein than bacteria.
Yeast has been considered advantageous because of its lower nucleic acid content than bacteria, but the protein to nucleic acid content ratio of yeast is substantially equal to that of bacteria. Thus, it may be said that it is most advantageous for improving productivity to use protein-rich bacterial cells as a protein source in feed and food.
Many reports have been published recently on the types of bacteria that can propagate microbial cells by utilizing methanol as a carbon source. Representative of such reports are the following: Preparation of Cells by Use of Bacteria Belonging to Pseudomonas by Kono et al. (Collection of Summaries of Lectures at Japan Agricultural Chemistry Society Congress 1H-23, 1970), Production of Cells by Use of Achromobacter methanolophia and Pseudomonas insneta by Kurasawa et al (Collection of Summaries of Lectures at Japan Agricultural Chemistry Society Congress 4I-31, 1970), and Production of Cells Using Pseudomonas methanolica by Terui et al (Collection of Summaries of Lectures at Japane Agricultural Chemistry Society Congress 2E-07, 1971).
Our search for bacteria that can provide useful microbial cells by utilizing methanol succeeded in isolating a new species of bacteria which allows most effective utilization of methanol and which has an extremely high growth rate and also high cell protein. On the basis of this success, we formulated the present invention which provides a method of producing microbial cells that can be utilized as a high-quality protein source by culture the new species of bacteria in a medium containing methanol as the carbon source.